bitcoin QT can sometimes use upwards of 1 GB of RAM.
With some custom settings to limit what gets held in memory, you could get this down to less than 10MB, but it would require a fair bit of work.
yes ... and it's very simple : decrease "cache" setting from 100MB to 10MB for example.
i must do this on my dedicated P2P server (because many others tasks work with bitcoin-QT in mode setting).